The present invention relates to an image scanner for obtaining image data for use in an electrophotographic image forming apparatus and an image forming apparatus having the image scanner.
An image scanner obtains image data by photoelectrically converting reflected light obtained by illuminating an object such as a sheet-like original, book, and three-dimensional object.
The image scanner includes an original holding portion (original document table) for holding an object and an illuminating unit has an illuminating lamp for illuminating the object put on the original document table, a charged-coupled device (CCD) sensor serving as an image scanning sensor, and an optical set (plural mirrors and reducing glass lenses) provided between the original document table and the CCD sensor to image the light reflected from the object on the CCD sensor. In the case of an analog electrophotographic apparatus in which an exposure mirror and a photosensitive drum are arranged instead of a CCD sensor to directly lead light reflected from an object to the photosensitive drum, an original document table (original holding portion), illuminating unit, and optical set have similar configurations.
When scanning the image of an object such as a book by the above image scanner, the entire region to be scanned does not always closely contact with an original document table. That is, when an object is a book, a portion of a binding margin frequently rises from the original document table.
In the case of the above system of moving an illuminating unit along an original document table and imaging light reflected from an object, i.e., image information on the object on a line sensor, the image information imaged on the line sensor is imaged on a light-receiving face of the line sensor at a focus fixed to an original document table. In other words, when the object set to the original document table has a portion which does not closely contact with the original document table, the image of the portion is unfocused and imaged on the line sensor.
In this case, a problem occurs that the sharpness of an output image signal is deteriorated and thereby, the image cannot be identified or image information cannot be obtained.
Though an image scanner for always imaging an input surface image on a photoelectric conversion element by moving an image imaged on a line sensor or the photoelectric conversion element and a lens in accordance with a distance between an original document table and an original is disclosed in Jpn. Pat. Appln.
KOKAI Publication No. 4-287561, a focus is generally adjusted every page or line. Therefore, also in the case of an image scanner provided with a mechanism capable of adjusting the focus of image information, a focus is controlled every page or line. Thus, in the case of scanning regions whose heights on an original document table are different from each other in a main scanning direction parallel to a longitudinal direction of a line sensor, a problem occurs that a uniform sharpness cannot be obtained because a focus is dislocated.